S R Silver1, R A Rinsky, S P Cooper, R W Hornung, D Lai. 1. Division of Surveillance, Hazard Evaluations, and Field Studies, National Institute for Occupational Safety and Health, 4676 Columbia Parkway, MS R-44, Cincinnati, Ohio, USA. ZRE4@cdc.gov
Abstract
BACKGROUND: Choice of follow-up time for an occupational cohort can influence risk estimates. We examined the effects of follow-up time on relative risk estimates for leukemia and multiple myeloma in a cohort of 1,845 rubber hydrochloride workers. MATERIALS AND METHODS: We generated standardized mortality ratios (SMRs) for yearly follow-ups, beginning each study in 1940 and increasing study end dates from 1950 through 1996. We used Cox proportional hazards modeling to explore the effects of follow-up time on the exposure-response relationship. RESULTS: The SMR for leukemia rose to 13.55 in 1961 and fell nearly monotonically to 2.47 by 1996. Cox modeling suggested interaction between cumulative exposure and time since exposure. A longer time to peak risk was seen for multiple myeloma. CONCLUSIONS: Because summary risk estimates change with follow-up time, exposure limits set using these estimates may not adequately protect workers. Consideration of appropriate follow-up time and use of more complex temporal models are critical to the risk assessment process.
BACKGROUND: Choice of follow-up time for an occupational cohort can influence risk estimates. We examined the effects of follow-up time on relative risk estimates for leukemia and multiple myeloma in a cohort of 1,845 rubber hydrochloride workers. MATERIALS AND METHODS: We generated standardized mortality ratios (SMRs) for yearly follow-ups, beginning each study in 1940 and increasing study end dates from 1950 through 1996. We used Cox proportional hazards modeling to explore the effects of follow-up time on the exposure-response relationship. RESULTS: The SMR for leukemia rose to 13.55 in 1961 and fell nearly monotonically to 2.47 by 1996. Cox modeling suggested interaction between cumulative exposure and time since exposure. A longer time to peak risk was seen for multiple myeloma. CONCLUSIONS: Because summary risk estimates change with follow-up time, exposure limits set using these estimates may not adequately protect workers. Consideration of appropriate follow-up time and use of more complex temporal models are critical to the risk assessment process.
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